Salt, hormones, and lymphatic system problems are the most factors in retaining water. Drinking water actually can help flush out your system. I know it's counterintuitive, but drinking water usually makes you less bloated.

It can make you lose weight. Because your stomach will be filled with water instead of only pizza. Water does not add to calories and will be released from your system in the next 1 hour. And only pizza will be digested. Pizza can increase your weight , so when you drink water you will eat less pizza and gain less weight.

I slice of pizza from a large pizza can be up to 650 calories, 2 slices would amount almost 67% of daily allotment of caloric intake. If you eat pizza regularly along with your normal breakfast, and lunch or dinner intake as well as snacks. Then yes the pizza would definitely piling on the weight. Water won't put any weight on you unless you're retaining fluids from too much salt intake or a physical/medical condition.

most probably this is an illusion. gaining weight is when the total day calories is more than it needs and no amount of water can change that. However, drinking water might make u look more fat because I ve some water in ur belly which will just need some cardio